Suppression of recJ exonuclease mutants of Escherichia coli by alterations in DNA helicases II (uvrD) and IV (helD)

Abstract

The recJ gene encodes a single-strand DNA-specific exonuclease involved in homologous recombination. We have isolated a pseudorevertant strain in which recJ mutant phenotypes were alleviated. Suppression of recJ was due to at least three mutations, two of which we have identified as alterations in DNA helicase genes. A recessive amber mutation, "uvrD517am," at codon 503 of the gene encoding helicase II was sufficient to suppress recJ partially. The uvrD517am mutation does not eliminate uvrD function because it affects UV survival only weakly; moreover, a uvrD insertion mutation could not replace uvrD517am as a suppressor. However, suppression may result from differential loss of uvrD function: mutation rate in a uvrD517am derivative was greatly elevated, equal to that in a uvrD insertion mutant. The second cosuppressor mutation is an allele of the helD gene, encoding DNA helicase IV, and could be replaced by insertion mutations in helD. The identity of the third cosuppressor "srjD" is not known. Strains carrying the three cosuppressor mutations exhibited hyperrecombinational phenotypes including elevated excision of repeated sequences. To explain recJ suppression, we propose that loss of antirecombinational helicase activity by the suppressor mutations stabilizes recombinational intermediates formed in the absence of recJ.